Two lenses of focal length f1 and f2, made from the same glass, are spaced by distance L. The effective focal length of the pair is related to refractive index n by
f^(-1) = (n-1)a + (n-1)b - L((n-1)^2)ab
where a,b are constants. Find the distance L required to make the effective focal length independent of wavelength, thereby making the lens combination achromatic.